The present invention relates generally to a device for determining if a load is present in an electronic system, and more particularly, to a circuit arrangement which determines if load current is present in a Thyristor device.
A silicon controlled rectifier (SCR), one type of "Thyristor", can be thought of as a switchable diode controlled by a positive gate signal. If connected in series with a supply voltage and load, and the supply voltage is less than the rated voltage of the device and no trigger current is applied to the gate, the Thyristor will remain off. If however, sufficient trigger signal is applied to the gate, the Thyristor's forward voltage will decrease to a value approximating the forward conduction drop of a diode, and the Thyristor will turn on, conducting as a diode. Once on, the Thyristor remains conducting (latches) regardless of the signal applied to the gate. For the Thyristor to be turned off, the anode-to-cathode current must be reduced below the holding current of the device.
Similarly, a triac operates in much the same way, but conducts in both directions when gated on (AC conduction), and accepts either polarity of gate signal.
Prior devices which are or can be used to detect the presence of a load in a system include resistors, current transformers, and Hall detectors; each of which has disadvantages: weight, size, cost, and/or added dissipation.
The present invention enables the detection of load current in a Triac or SCR (or similar Thyristor device) by monitoring the characteristic voltage found on the gate of the device.
Load status information can be used in various ways, for example, to turn off an output, to signal a load fault, etc. When incorporated into a control chip, the circuit of the present invention adds virtually no weight, requires no additional board space, and has negligible cost.
As an example of an application of the present invention, the circuit may be applied to a multi-channel chip that controls aircraft passenger reading lights. The chip removes power from burned-out lamps preferably within one second of their failure. This "lamp-fail reset" feature eliminates many relamping hazards with socket power still applied. Such hazards commonly include electrical shock, burned fingers, socket arcing, and multiple-surges during relamping. The present invention may be especially useful in connection with quartz halogen lamps due to the fact that such lamps operate at extremely high temperatures and pressures, and can even explode if haphazardly inserted into a live socket.